Wavelength Division Multiplexing (WDM) transmission systems have been used as a means to increase the transmission capacity. WDM allows a single optical fiber to send a plurality of optical signals each with a varying wavelength. Optical fiber amplifiers such as an Erbium Doped Fiber Amplifier (EDFA) simultaneously amplify a wide range of wavelengths. Due to the above simultaneous amplification, the combination of WDM and optical amplifiers enables economical transmission in a large capacity over a long distance with relatively simple structures.
To cover a long distance, optical amplifiers are used in succession for a series of spans or portions. Because the spans are not necessarily equal, the actual placement of the optical amplifiers is also not equidistant. Consequently, the amount of transmission loss varies for each span. For the above varying transmission loss, the input optical level to an optical amplifier is not constant for each optical amplifier, and the gain tilt occur among optical signals with different wavelengths.
The above gain tilt is accumulated over multiple optical amplifiers that are connected to a single optical fiber. At a final receiving unit, the gain tilt appears as a significant problem over a long distance. Since the gain tilt may exceed a predetermined reception dynamic range of a receiving unit, acceptable reception may not be possible. The optical signal levels during the transmission also vary among the wavelengths, and each optical signal receives a varying non-liner effect. Because of the above reasons, the optical signal waveform is affected for each wavelength.
The WDM optical gain transmission generally requires an optical signal at a high input level to be entered into an optical fiber in order to implement a long distance transmission system. It is concerned that the above described non-linear degrading effect on transmission would be further exaggerated due to the high optical input signal level. One of the non-linear effects is Stimulated Raman Scattering (SRS), which causes excessive loss or gain in optical signals. SRS is a non-linear optical process where a portion of the optical fiber input signal acts as stimulating light and energy moves from high-frequency signals to low-frequency signals by interacting with the low-frequency signals in the optical fibers. Although SRS occurs in all optical fibers, the effect depends upon the type of fibers and the frequency difference between the optical signals that are involved in energy transfer.
There is a number of factors for the amount of the above described energy transfer. The energy transfer is proportional to the sum of the output optical strength. The more wavelengths there are and the wider the wavelength range is in the WDM apparatus, the larger the amount of the energy transfers. As a result, the SRS effect is more typically seen. Further, the longer the transmission path is, the more apparent the SRS effect becomes. Under the influence of the SRS effect, the WDM transmission experiences a varying Optical Signal to Noise Ratio (OSNR) among the optical signals with a different wavelength since the wavelength signal level discrepancy occurs during transmission and the optical input level varies for each optical amplifier. Similar to the inter-wavelength gain tilt, the optical output strength also varies for each wavelength during transmission. As a result of the varying output strength, the reception waveform also experiences wavelength-dependent waveform distortions and transmission errors due to self phase modulation and wavelength dispersion as well as frequency chirping.
Prior art WDM systems had operated with a relatively small number of wavelengths and a relatively narrow range of frequencies in optical signals. For the above reasons, the effect of the inter-wavelength gain tilt and the SRS-induced signal level discrepancy had not been significant. Contrarily, as the communication traffic has increased in the recent years, the WDM systems have been demanded to cover a longer distance, to accommodate a larger number of frequency signals and to widen a range of the frequency. As a result of the added requirements, the effect of the inter-wavelength gain tilt and the SRS-induced signal level discrepancy can be no longer ignored. It has been reported in academics that transmission becomes disabled due to the above described increased SRS effects when the initial design capacity for the wavelengths is reached in the WDM systems.
To deal with the above described undesirable effects in the WDM systems, prior art has considered the discrepancy in gain tilt in optical amplifiers and the wavelength-dependent loss in the transmission path. For example, Japanese Patent Publication Hei 8-223136 discloses a method of minimizing the gain tilt in the optical amplifiers to obtain the minimal OSNR. Another example is Japanese Patent Publication Hei 11-55182, which discloses a method of minimizing the optical signal level discrepancy and the OSNR at a receiving end. The above prior art methods are useful for the situations where the optical signal loss amount is equal among the spans in the transmission path. Alternatively, the above prior art is also useful if the optical signal loss and the optical amplifier gain tilt are not affected by a number of wavelength frequencies in the optical signal to be inputted in an optical amplifier. Unfortunately, the above described prior art technologies do not account for the situations where the optical signal loss amount is not equal among the spans in the transmission path. The above prior art technologies also fail consider that the optical signal output strength is different among a number of wavelength frequencies or ranges of frequencies in the optical signal.
Japanese Patent Publication 2000-183818 discloses the method of adjusting optical signal strength at a transmission side for each wavelength by pre-emphasis so as to stabilize the OSNR at a receiving side. However, the above method depends upon a range and a number of frequencies, and the discrepancy or tilt in signal level among the optical signals may exceed the pre-emphasis guaranty.
For the above described reasons, it is desirable to guarantee the OSNR among the optical signals at the receiving terminal in the WDM systems. In other words, it is desirable to guarantee an equal optical signal loss amount among the spans in the transmission path so that the optical amplifier gain tilt is not affected among a number of wavelength frequencies in the optical signal. Furthermore, it is also desirable to guarantee the signal level among frequencies due to SRS during the optical fiber transmission.